The availability of safe, low-cost, long cycle-life, and efficient energy storage devices is essential to increased use of renewable energy and environmentally friendly electric vehicles (EVs). Rechargeable lithium-ion (Li-ion), lithium metal, lithium-sulfur, and Li metal-air batteries are considered promising power sources for electric vehicle (EV), hybrid electric vehicle (REV), and portable electronic devices, such as lap-top computers and mobile phones. Lithium as a metal element has the highest lithium storage capacity (3,861 mAh/g) compared to any other metal. Hence, in general, Li metal batteries (having a lithium metal anode) have a significantly higher energy density than lithium-ion batteries (having a graphite anode with a theoretical specific capacity of 372 mAh/g).
Historically, rechargeable lithium metal batteries were produced using non-lithiated compounds, such as TiS2, MoS2, MnO2, CoO2, and V2O5, as the cathode active materials, which were coupled with a lithium metal anode. When the battery was discharged, lithium ions were transferred from the lithium metal anode to the cathode through the electrolyte, and the cathode became lithiated. Unfortunately, upon repeated charges and discharges, the lithium metal resulted in the formation of dendrites at the anode that ultimately penetrated through the separator to reach the cathode, causing internal shorting, thermal runaway, and explosion. As a result of a series of accidents associated with this problem, the production of these types of secondary batteries was stopped in the early 1990's giving ways to lithium-ion batteries.
Even now, cycling stability and safety concerns remain the primary factors preventing the further commercialization of Li metal batteries (e.g. Lithium-sulfur and Lithium-transition metal oxide cells) for EV, HEV, and microelectronic device applications. Again, cycling stability and safety issues of lithium metal rechargeable batteries are primarily related to the high tendency for Li metal to form dendrite structures during cycling or overcharges, leading to internal electrical shorting and thermal runaway. This thermal runaway or even explosion is caused by the organic liquid solvents used in the electrolyte (e.g. carbonate and ether families of solvents), which are unfortunately highly volatile and flammable.
Parallel to these efforts and prompted by the aforementioned concerns over the safety of earlier lithium metal secondary batteries led to the development of lithium-ion secondary batteries, in which pure lithium metal sheet or film was replaced by carbonaceous materials (e.g. natural graphite particles) as the anode active material. The carbonaceous material absorbs lithium (through intercalation of lithium ions or atoms between graphene planes, for instance) and desorbs lithium ions during the re-charge and discharge phases, respectively, of the lithium-ion battery operation. The carbonaceous material may comprise primarily graphite that can be intercalated with lithium and the resulting graphite intercalation compound may be expressed as LixC6, where x is typically less than 1, implying a relatively low anode specific capacity (theoretically 372 mAh/g of graphite, but practically 300-360 mAh/g). Since the cathode specific capacity is typically in the range of 130-200 mAh/g, the energy density of the resulting lithium-ion cell is typically in the range of 150-200 Wh/g.
Although lithium-ion (Li-ion) batteries are promising energy storage devices for electric drive vehicles, state-of-the-art Li-ion batteries have yet to meet the cost, safety, and performance targets. In particular, the same flammable solvents previously used for lithium metal secondary batteries are also used in most of the lithium-ion batteries. Despite the notion that there is significantly reduced propensity of forming dendrites in a lithium-ion cell (relative to a lithium metal cell), the lithium-ion cell has its own intrinsic safety issue. For instance, the transition metal elements in the lithium metal oxide cathode are highly active catalysts that can promote and accelerate the decomposition of organic solvents, causing thermal runaway or explosion initiation to occur at a relatively low electrolyte temperature (e.g. <200° C., as opposed to normally 400° C. without the catalytic effect).
As a totally distinct class of energy storage device, sodium batteries have been considered as an attractive alternative to lithium batteries since sodium is abundant and the production of sodium is significantly more environmentally benign compared to the production of lithium. In addition, the high cost of lithium is a major issue.
Sodium ion batteries using a hard carbon-based anode (Na-carbon intercalation compound) and a sodium transition metal phosphate as a cathode have been described by several research groups: Zhuo, X. Y. Wang, A. P. Tang, Z. M. Liu, S. Gamboa, P. J. Sebastian, Journal of Power Sources 160 (2006) 698; J. Barker, Y. Saidi, J. Swoyer, US Patent Application US2005/0238961, 2005; J. Barker; M. Y. Saidi, and J. Swoyer, “Sodium Ion Batteries,” U.S. Pat. No. 7,759,008 (Jul. 20, 2010 and J. F. Whitacre, A. Tevar, and S. Sharma, “Na4Mn9O18 as a positive electrode material for an aqueous electrolyte sodium-ion energy storage device,” Electrochemistry Communications 12 (2010) 463-466.
However, these sodium-based devices exhibit even lower specific energies and rate capabilities than Li-ion batteries. These conventional sodium-ion batteries require lithium ions to diffuse in and out of a sodium intercalation compound at both the anode and the cathode. The required solid-state diffusion processes for sodium ions in a sodium-ion battery are even slower than the Li diffusion processes in a Li-ion battery, leading to excessively low power densities. Instead of hard carbon or other carbonaceous intercalation compound, sodium metal may be used as the anode active material in a sodium metal cell. However, the use of metallic sodium as the anode active material is normally considered undesirable and dangerous because of dendrite formation, interface aging, and electrolyte incompatibility problems. Most significantly, the same flammable solvents previously used for lithium secondary batteries are also used in most of the sodium metal or sodium-ion batteries.
Hence, a general object of the present invention is to provide an electrolyte system for a rechargeable Na metal cell, K metal cell, hybrid Na/K metal cell, Na-ion cell, K-ion cell, or hybrid Na/K cell that exhibits a high energy density, high power density, long cycle life, and no danger of explosion due to the use of a safe, non-flammable, quasi-solid electrolyte. The invention also provides a rechargeable non-lithium alkali metal or alkali-ion cell containing such a safe electrolyte system. This cell includes the Na or K metal secondary cell, Na-ion cell, K-ion cell, or a non-lithium alkali metal hybrid cell, wherein at least one electrode operates on Na or K insertion or intercalation.
A specific object of the present invention is to provide a rechargeable non-lithium alkali metal battery that exhibits an exceptionally high specific energy or high energy density and a high level of safety. One specific technical goal of the present invention is to provide a safe Na- or K-metal based battery having a long cycle life and a cell specific energy greater than 200 Wh/Kg, preferably greater than 300 Wh/Kg, and more preferably greater than 400 Wh/Kg (all based on the total cell weight).
A specific object of the present invention is to provide a rechargeable non-lithium alkali metal cell based on rational materials and battery designs that overcome or significantly reduce the following issues commonly associated with conventional alkali metal cells: (a) dendrite formation (internal shorting due to sharp dendrite penetrating the separator to reach the cathode); (b) extremely low electric and ionic conductivities of Na intercalation compound in the cathode, requiring large proportion (typically 10-30%) of non-active conductive fillers and having significant proportion of non-accessible or non-reachable cathode active material); and (c) short cycle life.
Another object of the present invention is to provide a simple, cost-effective, and easy-to-implement approach to preventing potential Na metal dendrite-induced internal short circuit and thermal runaway problems in various Na metal and Na-ion batteries.